1. Field of the Invention
The present invention relates to a photodiode and a phototransistor including the photodiode.
2. Description of the Related Art
A photodiode has been widely used as a pickup for reading information from an image sensor or an optical disk such as a CD or a DVD.
A typical photodiode in the prior art has a structure, in which, for example, an N type impurity region and an electrode contact region of an N+ type (a high concentration N type) having an electrode connected to the inside of the N type impurity region are provided at a surface of a P+ type (a high concentration P type) substrate. In such a photodiode, light incident from the surface of the N type impurity region is optoelectronically transduced at a junction surface between the N type impurity region and the P+ type substrate. The resultant electric charge is transmitted out of the electrode connected to the electrode contact region as a signal representing a light reception intensity.
When a power source, OA equipment, or other electronic component is arranged near the above-described photodiode, the photodiode might not output the signal representing the light reception intensity but instead a noise signal caused by an adverse influence of an electromagnetic wave (i.e., an electromagnetic noise) generated by the peripheral component. When the photodiode outputs the noise signal by the adverse influence of the electromagnetic wave in the case where, for example, the photodiode is incorporated in an image sensor, an undesired image such as a stripe pattern appears on an image reproduced from an output from the image sensor.
In view of this, a P+ type diffused region 94 is provided at an outermost surface of a P+ type substrate 93 having an N type impurity region 91 and an N+ type electrode contact region 92 so as to surround the electrode contact region 92, and further, the P+ type diffused region 94 is connected to a ground potential, as shown in FIG. 5. The impedance of the P+ type diffused region 94 becomes low by connecting the P+ type diffused region 94 to the ground potential so that the P+ type diffused region 94 exhibits a shield effect against an electromagnetic wave. As a consequence, even if the electromagnetic wave intrudes at the surface of the photodiode through an electronic component arranged near the photodiode, the electromagnetic wave can be shielded in the P+ type diffused region 94, thereby preventing any generation of an electric charge caused by the electromagnetic wave.
Since the electrode contact region 92 and the P+ type diffused region 94 are arranged adjacent to each other, a pn junction of a high impurity concentration is formed, and therefore, a tunnel current flows between the electrode contact region 92 and the P+ type diffused region 94 caused by Zener breakdown. In addition, the electric charge is concentrated at the junction between the electrode contact region 92 and the P+ type diffused region 94 thereby raising another problem of a reduced withstand voltage at the junction. Thus, the electrode contact region 92 and the P+ type diffused region 94 must be arranged to have a certain interval therebetween.
However, if the electrode contact region 92 and the P+ type diffused region 94 are separated from each other, the electromagnetic wave cannot be shielded in a region defined between the electrode contact region 92 and the P+ type diffused region 94 from entering into the N type impurity region 91 through the above-described region, thereby raising a possibility that a signal (i.e., an electric charge) caused by the electromagnetic wave may be output from the electrode connected to the electrode contact region 92.